Method of fabricating quantum bridges by selective etching of superlattice structures

ABSTRACT

A quantum bridge structure including wires of a semiconductor material such as silicon which are formed by selectively etching a superlattice of alternating layers of at least two semiconductor materials. The quantum bridge is useful as a photo emission device, a photo detector device, and a chemical sensor. The wires exhibit improved electrical conduction properties due to decreased Coulomb scattering.

This is a division of application Ser. No. 08/384,166, filed Feb. 6,1995, now U.S. Pat. No. 5,539,214.

BACKGROUND OF THE INVENTION

This invention relates generally to bandgap engineered semiconductorstructures and more particularly the invention relates to structureshaving dimensions sufficiently small so as to confine carriers locatedtherein and resulting in novel electronic and optical properties.

There has been considerable attention paid to quantum wires since theirproposal by H. Sakaki in the Jap. J. Appl. Phys. 19, L735-738 (1980).More recently, Canham in Appl. Phys. Lett. 57, 1046 (1990) has observedstrong photoluminescence in anodically etched silicon, or poroussilicon. The structure of this silicon resembles a sponge, composed of amyriad of silicon quantum dots or intertwined quantum wires.

Considerable work has been done in light emission from silicon.Currently, group IV semiconductors (silicon and germanium) are not usedin lasers and light emitting diodes even though the materials are widelyused for their electrical properties. A silicon based light emittingdiode or laser would be a tremendous boon to the semiconductor industrywith speed significantly improved over those of electrical conduction.The current research in silicon light emission has been in the area ofporous silicon of the type noted above.

It has also been shown that quantum wires may show significantimprovements in transport properties over bulk semiconductors due todecreased Coulomb scattering. Many other novel effects related to thequantum confinement have been reported such as non-local bendresistance, the quenching of the Hall effect, and the oscillatorybehavior of capacitance and conductivity. Further, the opticalproperties of porous silicon can be combined with the improvedelectrical property to function as sensors.

Heretofore, quantum wire superlattices in gallium arsenide/aluminumgallium arsenide have been proposed for 8-20micrometer photodetectors. Atheoretical study of p-i-n GaAs/AlGaAs quantum wire detectors proposesthat they may perform better than bulk devices due to increasedabsorption of photons and the higher carrier mobilities therein. See D.L. Crawford et al., Applied Physics Letters, 58 (1991), p. 1629.

The present invention is directed to the fabrication of a quantum bridgestructure whose length, width, and height can be controlled byprocessing parameters. The resulting structure can exhibitphotoluminescence and electroluminescence which allows the device tofunction in a light emitter, light detector, conductive wire, and asensor mode.

SUMMARY OF THE INVENTION

In accordance with the invention, quantum bridges are provided by theselective etching of superlattice structures. By carefully controllingthe process parameters, the physical dimensions as well as thephotoluminescence and electroluminescence of the bridge structure can bereadily varied. The resulting structure can then be employed in variousapplications of light emission, light detection, and electricalconduction.

In accordance with a preferred embodiment, the process in fabricatingthe quantum bridge structure begins with a supporting substrate ofsilicon or other material on which a silicon buffer layer can beepitaxially grown. For stress relief, a graded silicon to Si_(x)Ge_(1-x) buffer layer is then epitaxially grown on the silicon bufferlayer. A superlattice structure of alternating silicon and germaniumlayers is then grown on the silicon/germanium buffer layer.

The bridge structure is then formed by selectively etching trenchesthrough the superlattice structure using E-beam or I-beamphotolithography and plasma etching. Germanium material between thesilicon wires is then removed by selective wet etching. Contacts ateither end of the wire structure are formed by high energy ionimplantation into supporting superlattice columns.

The invention and objects and features thereof will more readilyapparent from the following detailed description and dependent claimswhen taken with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a quantum bridge structure in accordance withone embodiment of the invention.

FIG. 2 is a side view of the quantum bridge structure of FIG. 1.

FIG. 3 is a cross-section view of the quantum bridge structure FIGS. 1and 2 taken along the line 3--3 in FIG. 2.

FIG. 4 is a flow diagram of a process for fabricating the quantum bridgestructure in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, FIGS. 1, 2, and 3 are a top view, side view,and cross-section view of a quantum bridge structure in accordance withone embodiment of the invention. The structure is fabricated on asupporting substrate 10 of silicon, or a substrate on which a bufferlayer of silicon can be grown. The bridge structure includes a pluralityof silicon wires 12 which are supported at either end by superlatticegermanium-silicon columns 14. Electrical contacts to the quantum wires12 are provided by implanted dopant ions in the superlattice supports 14with suitable metal contacts 16 provided on the superlattice supports14.

The quantum bridge is a mesa structure on the supporting substrateformed by removing by etching surrounding material. The quantum wires 12are fabricated by etching parallel trenches perpendicular to the planeof the superlattice layers and then removing by preferential etchant thealternating layers of germanium in the superlattice. Thus, in the topview of FIG. 1 the trenches down to the substrate are visible while theside view of FIG. 2 illustrates the superlattice layering of thestructure with the wires in the middle portion of the structure formedby removing selectively etched germanium between the silicon wires. Inthe unetched areas the superlattice remains intact as grown to providesupports and low resistance paths to the silicon wires.

FIG. 3 shows a cross-section of the quantum wire structure taken alongthe line 3--3 of FIG. 2. The number of rows and columns of wires can bevaried as can be the cross-sectional shape and dimensions of the wires.While the wires are illustrated as uniformly spaced, the spacing betweenwires both vertically and horizontally is readily varied by processcontrol. The vertical height of the wires, vertical spacing betweenwires, and number of rows are varied by changing the thicknesses andnumbers of layers epitaxially deposited. Horizontal width of the wires,horizontal spacing between wires, and the number of wires are varied bychanging the width, spacing, and the number of trenches. Further, allwires need not have the same dimensions. Varying dimensions within onestructure can lead to possible additional beneficial effects, such asluminescence over a broader wave length spectrum.

FIG. 4 is a process flow in the fabrication of the quantum bridgestructure in accordance with one embodiment of the invention. Initially,the superlattice layers are grown on a supporting substrate by usingmolecular beam epitaxy. The supporting substrate includes a siliconsubstrate or a silicon buffer layer of 3,000 Å on a supportingsubstrate. Next a 2.5 micron silicon to Si.sub..5 Ge.sub..5 gradedbuffer is formed on the silicon buffer layer. A 1.0 micron thickSi.sub..5 Ge.sub..5 buffer is then formed on the graded buffer.Thereafter, alternating layers of silicon and germanium are epitaxiallygrown with each layer being on the order of 500 Å in thickness.Alternatively, the alternating layers can comprise the same materialsilicon, for example, with adjacent layers being distinguished bydoping. A layer of undoped silicon can be preferentially etched overdoped silicon. For example, ethylenediamine-pyrocatechol-water (EPW)etches highly doped silicon much more slowly than lower doped or undopedsilicon.

A cap layer of silicon oxide or silicon oxide/silicon nitride is thenformed on the top silicon layer as a hard mask, and a trench pattern isdefined thereon using E-beam or I-beam photolithography in developing aphoto resist. A tight pitch array of lines can thus be etched throughthe cap layer. Thereafter, slots are etched through the underlyingsuperlattice structure using a suitable dry etch which can include anumber of etching species including CF₄, CHF₃, SF₆ and combinationsthereof with O₂.

Thereafter, the alternate layers of germanium between the silicon layersare removed by selective wet etching which can consist of a combinationof HF, H₂ O₂, and H₂ O which etches germanium considerably faster thansilicon. For a superlattice that consists of regions of alternatelydoped materials, an etchant with an etch rate that is dependent ondoping will be employed. One example of this is EPW(ethylenediamine-pyrocatechol-water) which etches highly doped siliconmuch more slowly than lower doped silicon.

The electrical contacts through the superlattice structures 14 at eitherend of the bridge are formed by high energy ion implantation through thesuperlattice structure, preferably before the etching of the germaniumlayers. Metal bonding contacts 16 are then provided on the top surfacesof the superlattice structures 14.

The resulting bridge structure has many applications as a light emitter,a light sensor, a chemical sensor, and electrical current conductor. Adistribution of wave length emissions can be controlled by adjustingpattern dimensions and by surface treatment. For example, luminescenceincreases with a passivating oxide and almost completely disappearsafter a dip in hydrofluoric acid to remove the oxide. The change inquantum properties, electrical and light emission and detection, due tochemical adsorption makes the bridge structure suitable for use as achemical sensor. The precise control over small dimensions of thesilicon wires in the bridge structure permits a desired light emissionor obstruction of species on a pretreated surface which permits thestructure to function as a detector for that species.

While the invention has been described with reference to a preferredembodiment of germanium and silicon superlattice, the invention hasapplicability to other materials including different doped layers whichcan be preferentially etched and to III-V materials. Thus, while theinvention has been described with reference to specific embodiments, thedescription is illustrative of the invention and is not to be construedas limiting the invention. Various modifications and applications mayoccur to those skilled in the art without departing from the true spiritand scope of the invention as defined by the appended claims.

What is claimed is:
 1. A method of fabricating a quantum bridgestructure comprising the steps ofa) providing a supporting substratehaving a monocrystalline surface of a first semiconductor material, b)depositing a buffer layer on said monocrystalline surface for stressrelaxation, c) alternately depositing layers of said first semiconductormaterial and a second semiconductor material over said buffer layer toform a superlattice structure, d) etching trenches through saidsuperlattice structure to form parallel rows of superlattice materialand to form end support members for parallel wires, and e) selectivelyetching said second semiconductor material from said parallel rowsthereby forming parallel wires of said first semiconductor material,said parallel wires including a plurality of horizontally spaced wiresparallel to said monocrystalline surface and a plurality of verticallystacked wires.
 2. The method as defined by claim 1 wherein said firstsemiconductor material is a III-V semiconductor and said secondsemiconductor material is a III-V semiconductor.
 3. The method asdefined by claim 1 wherein said first semiconductor layer is asemiconductor material having a first dopant therein and said secondsemiconductor layer is said semiconductor material with a second dopanttherein which differs from said first dopant in at least one of dopanttype and dopant concentration.
 4. The method as defined by claim 1wherein said substrate comprises silicon.
 5. The method as defined byclaim 1 wherein said substrate includes a surface layer of silicon. 6.The method as defined by claim 1 wherein said first semiconductormaterial is silicon and said second semiconductor material is germanium.7. The method as defined by claim 6 wherein said buffer layer is a gradelayer of Si_(x) Ge_(1-x), where x is the mole ratio.
 8. The method asdefined by claim 6 wherein said step of etching trenches includes plasmaetching and said step of selectively etching includes anisotropic wetetching.
 9. The method as defined by claim 8 wherein step c) formsalternating layers of silicon and germanium each on the order of 20Å-2000 Å in thickness.
 10. The method as defined by claim 9 wherein stepb) forms a graded buffer layer approximately 2.5 μm in thickness and aSi.sub..5 Ge.sub..5 layer approximately 1.0 μm in thickness.
 11. Themethod as defined by claim 1 and further including the step ofimplanting dopant ions in said end support members for electricalcontact to said parallel wires.
 12. The method as defined by claim 11and further including the step of depositing contact material on saidend support members for contacting said implanted dopant ions.
 13. Themethod as defined by claim 1 and further including the step of:f)treating surfaces of said wires to alter quantum properties thereof. 14.The method as defined by claim 13 wherein step f) includes forming anoxide layer on said surfaces.